The Sanzu River (三途の川,Sanzu no Kawa, literally "River of Three Ways to Cross", also in British English translation River Sanzu) is the mist-shrouded river that dead souls must cross in order to reach Higan, where they will await judgment. Part of the riverbank is accessible from Gensokyo, even though the river's source, mouth and opposite bank lie outside it. Only the dead can cross the river on a shinigami's boat if they can afford to pay; everything except shinigami and their boats will sink.

Komachi Onozuka may be seen ferrying souls here when she isn't slacking off. In Japanese Buddhist tradition, the Sanzu is said to have three crossings which vary in perilousness depending on the amount of sin a soul committed during his or her lifetime (hence the name). However, this concept is not used in Touhou: instead, Komachi uses her power of distance manipulation to vary the width of the river according to the souls' sinfulness.